1. Field of the Invention
The invention relates to a gas detonation spraying apparatus. More particularly, the invention relates to a spraying apparatus which uses an explosive mixture of gases to propel a powder material against a surface.
2. State of the Art
Gas detonation spraying devices are well known in the art. They are typically used to apply coatings to machine parts. However, they may also be used for grinding powder materials or for cleaning or heating surfaces. They may also be used for welding or perforating non-metallic materials. In addition, they can be used to obtain powders having new physical properties or for producing structural changes in the surface layer of a material. These apparatus generally include a propulsion barrel and a combustion chamber. A mixture of combustible gases is introduced into the combustion chamber and a powder is introduced into the propulsion barrel. When the gases are detonated in the combustion chamber, the detonation wave is directed into the propulsion barrel and propels the powder through the barrel to exit the barrel as a spray. The process is repeated at intervals to produce repeated bursts of powder spray.
Gas detonation spraying devices present several engineering challenges in order to assure efficiency while maintaining safety. While a mixture of oxygen and acetylene produces a powerful detonation wave, acetylene is very dangerous. It is possible for acetylene m detonate even in the absence of oxygen. Moreover, acetylene is relatively expensive. It has therefore been known to use less combustible gases such as natural gas, proparte, butane, or a mixture of proparte and butane. These gases are safer and less expensive than acetylene. However, they are more difficult to detonate. Sprayers which use these less combustible gases typically include special devices to accelerate the gas mixture from slow combustion to detonation. These special devices dramatically increase the overall dimensions of the sprayer and also lower its capacity. Since it takes more time to detonate the gas mixture, the firing frequency of the sprayer is limited. In addition, the proportions of gas, oxygen, and powder must be strictly controlled within narrow tolerances in order for the sprayer to operate with acceptable efficiency.
Another important aspect of gas detonation spraying devices is that they can backfire. Backfiring is typically prevented by intermittently injecting nitrogen into the combustion chamber. This requires the fuel supply to be injected intermittently as well. The intermittent injection of nitrogen and fuel requires the constant opening and closing of valves which complicates the device and makes it more prone to failure. The firing frequency is also consequently limited. Moreover, the presence of nitrogen in the combustion chamber reduces the velocity of the detonation wave which has a negative affect on the quality of sprayed coatings.
A typical gas detonation spraying device is disclosed in U.S. Pat. No. 4,669,658 to Nevgod et al., the complete disclosure of which is incorporated herein by reference. The device disclosed by Nevgod et al. includes a main barrel having a combustion chamber with a spark plug and a gas supply system which includes a gas heater. A main pipe is provided at one end of the barrel and the main pipe houses the spark plug. Annular grooves in the main pipe and in the barrel enhance efficient heat transfer between the walls of the barrel and the entering gases. Gases are heated to a temperature approximating self-ignition during a first operating cycle and thereafter are ignited more easily. A buffer unit having spiral gas conduits is provided upstream of the gas heater to prevent backfire. However, the mount of gas fed into the apparatus is limited by the diameters of the gas heater. Thus, the stable operation of the device is limited to a specific gas feeding rate and thus the proportion of gases mixed. Moreover, the gas heater and the buffer are complicated and expensive to construct.